1. Field of the Invention
The present invention generally relates to lithography, and more particularly to patterning device inspection.
2. Related Art
A lithographic apparatus is a machine that applies a pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, can be used to create a circuit pattern formed on an individual layer of the IC. The pattern can be transferred onto the target portion (e.g., comprising part of, one, or several dies) of the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging the pattern onto a layer of radiation-sensitive material (resist) on the substrate. In general, a single substrate will contain a network of adjacent target portions that are patterned. Known lithographic apparatus include steppers, in which each individual target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To image smaller features, it has been proposed to use extreme ultraviolet radiation (EUV) as exposure radiation in a lithographic apparatus. EUV radiation needs the exposure radiation beam path in the apparatus to be evacuated to avoid absorption of the exposure radiation. EUV lithographic apparatus use a patterning device, such as a mask or a reticle, to impart a pattern into a beam of EUV radiation. Such patterning devices are highly susceptible to contamination, such as particulate contamination, which provides image defects in the pattern imparted into the EUV radiation beam. The image defects at least degrade, and in some cases, can destroy the performance of an IC manufactured with a contaminated reticle. The image defects reduce yield of the lithographic apparatus. Therefore, inspecting patterning devices for contamination is important to maintain and improve yield of the lithographic apparatus.
Conventionally, masks are inspected for contamination using two methods, absolute detection and a comparison technique. Absolute detection measures a scattered signal from a probe beam. Analysis of the scattered signal may indicate a particular type and size of particulate contamination. However, absolute detection is not able to accurately distinguish between a signal produced by scattering from a reticle pattern and a signal produced by scattering from contamination on the reticle pattern. This problem occurs because contamination can be composed of any material having dimensions down to a molecular level, which unfortunately includes dimensions of a reticle's absorber structures. As a result, when using absolute detection, a scattered signal from a particle on the reticle's surface is indistinguishable from a signal scattered by the reticle's absorber structures. Thus, absolute detection is often relegated to inspecting non-patterned surfaces, such as a back-side of a reticle or a pellicle.
Another conventional method for reticle inspection uses comparison techniques. Comparison techniques include two common methods for inspecting patterned surfaces of a reticle. The first traditional comparison technique is commonly known as “die-to-die” comparison. This technique contemporaneously compares a first pattern on a substrate to a second pattern, similar to the first pattern, that is also located on the substrate. This technique essentially compares a first optical signal from a first pattern on the reticle with a second optical signal from a second pattern on the same reticle. Problems with this technique include that it cannot be performed when the reticle contains only one pattern or only a collection of unique patterns. Further, variations in absorber structure sizes and variations in critical dimensions between the first and second patterns can complicate the particle detection process and undesirably limit inspection accuracy.
The second conventional comparison technique compares a pattern on a tangible reticle with a computer-generated theoretical reticle layout design from which the tangible reticle was fabricated. Problems with this technique include that errors in a reticle fabrication process, such as those stemming from critical dimension control, are not represented in the computer-generated theoretical reticle layout design. These errors complicate the comparison technique by indicating differences between the tangible reticle and the computer-generated theoretical reticle layout design that are false positive indications of contamination. Thus, this comparison technique inherently produces errors that complicate the particle detection process and undesirably limit inspection accuracy.
Therefore, the conventional systems and methods for inspecting a reticle suffer from significant disadvantages.